A flight management system FMS aboard an aircraft fulfils notably the function of predicting the progress of a flight plan. This involves predictions at each point of time of passby, speed and fuel consumed.
These predictions are extremely dependent notably on the meteorological conditions and, more particularly, on the winds encountered. The predictions of the FMS are based at one and the same time on a model entered by the company (arising from meteorological modelling) and on a measurement made by sensors aboard the aircraft.
Within the framework of the future concepts of air traffic management, and solutions envisaged for increasing airspace capacity while guaranteeing the safety level demanded, it is envisaged that aircraft will increasingly communicate these predictions to the ATC (Air traffic control).
By virtue of the information regarding passby time, the ATC ascertains the margins of manoeuvre of each aircraft and can give them directives (ATC clearances) to keep the traffic flowing and increase the overall capacity of the air transport system.
The ATC is envisaging notably drawing up ever more RTA (for Required Time of Arrival) contracts with aircraft to ensure the sequencing of traffic arrivals of large airport hubs. An RTA contract imposes an airport arrival time on an aircraft. The aim is to guarantee an approach flow which is close to the maximum capacity of the landing runway, and acceptable from an air traffic control point of view, while reducing to the maximum the holding circuits on arrival.
In order that the tools for scheduling and regulating traffic on the ground can provide aircraft with precise time constraints, it is necessary that aircraft be able to provide the ATC with reliable predictions about the passby times at the points of the flight plan and that they be in a position to adhere to them precisely. Indeed, the ground tools are based on the estimations of passby times at characteristic points in order to perform their flow optimization. Non-compliance with the passby times destabilizes the system.
As regards the RTA clearance, the aeroplane provides the ATC with the minimum time ETAmin (for Estimated Time of Arrival Minimum) and maximum time ETAmax (for Estimated Time of Arrival Maximum) at which the aeroplane can overfly a precise point as a function of its capabilities, of its configuration and of the meteorological conditions.
In order that the ATC has a margin of manoeuvre in the sequencing of aircraft and that this margin of manoeuvre is truly effective, it is important that each aeroplane provides the largest possible time window ETAmin-ETAmax, while guaranteeing its capability to be able to comply with its predictions in this time window (i.e. this window must be reliable).
FIG. 1 illustrates an ETAmin-ETAmax time window and the uncertainty margins taken by the FMS. The FMS calculates reliable margins in the predictions ETAmin and ETAmax communicated to the ATC as a function of the meteorological unknowns which may arise. In practice, the FMS calculates a first time window, termed a raw RTA window, on the basis of raw forecasts of raw minimum arrival time ETAminRaw and of raw maximum arrival time ETAmaxRaw. The FMS thereafter calculates a second time window intended to be communicated to the ATC. The second window corresponds to the first window from which two margins are removed. A first margin MarginMin takes into account a headwind of 10 knots and a second margin MarginMax takes into account a tailwind of 10 knots. The value of 10 knots is related to the precision of the meteorological forecasts announced by the provider of these forecasts which are precise to within 10 knots, 95% of the time. It is useful to take penalizing margins on these data to calculate the minimum and maximum arrival times and communicate them to the ATC.
But these margins are not entirely satisfactory since they are static and do not make it possible to take precise account of the vagaries encountered by an aircraft.
Current systems take a constant, conservative margin, presumed to be valid in most cases. This margin is consequently fairly high, and therefore decreases the time window unnecessarily.